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Date:   Wed, 11 Nov 2020 13:43:57 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Liang, Kan" <kan.liang@linux.intel.com>
Cc:     Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>, mingo@redhat.com,
        acme@kernel.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, eranian@google.com, ak@linux.intel.com,
        dave.hansen@intel.com, kirill.shutemov@linux.intel.com,
        benh@kernel.crashing.org, paulus@samba.org,
        David Miller <davem@davemloft.net>, vbabka@suse.cz,
        willy@infradead.org
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
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On Wed, Nov 11, 2020 at 12:22:46PM +0100, Peter Zijlstra wrote:

> Trying to match the Code: to PageHuge as generate here makes this the
> PageCompound() test burning, not even compound_head() going bad.
> 
> must ponder more...

Oooh.. does this help?

---
 kernel/events/core.c | 81 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 49 insertions(+), 32 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d2f3ca792936..3b42576c99f1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7015,65 +7015,82 @@ static u64 perf_virt_to_phys(u64 virt)
  */
 __weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr)
 {
+#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
 	struct page *page;
-	pgd_t *pgd;
-	p4d_t *p4d;
-	pud_t *pud;
-	pmd_t *pmd;
-	pte_t *pte;
-
-	pgd = pgd_offset(mm, addr);
-	if (pgd_none(*pgd))
+	pgd_t *pgdp, pgd;
+	p4d_t *p4dp, p4d;
+	pud_t *pudp, pud;
+	pmd_t *pmdp, pmd;
+	pte_t *ptep, pte;
+
+	pgdp = pgd_offset(mm, addr);
+	pgd = READ_ONCE(*pgdp);
+	if (pgd_none(pgd))
 		return 0;
 
-	p4d = p4d_offset(pgd, addr);
-	if (!p4d_present(*p4d))
+	p4dp = p4d_offset(&pgd, addr);
+	p4d = READ_ONCE(*p4dp);
+	if (!p4d_present(p4d))
 		return 0;
 
-	if (p4d_leaf(*p4d))
+	if (p4d_leaf(p4d))
 		return 1ULL << P4D_SHIFT;
 
-	pud = pud_offset(p4d, addr);
-	if (!pud_present(*pud))
+	pudp = pud_offset(&p4d, addr);
+	pud = READ_ONCE(*pudp);
+	if (!pud_present(pud))
 		return 0;
 
-	if (pud_leaf(*pud)) {
+	if (pud_leaf(pud)) {
 #ifdef pud_page
-		page = pud_page(*pud);
-		if (PageHuge(page))
-			return page_size(compound_head(page));
+		if (!pud_devmap(pud)) {
+			page = pud_page(pud);
+			if (PageHuge(page))
+				return page_size(compound_head(page));
+		}
 #endif
 		return 1ULL << PUD_SHIFT;
 	}
 
-	pmd = pmd_offset(pud, addr);
-	if (!pmd_present(*pmd))
+	pmdp = pmd_offset(&pud, addr);
+	pmd = READ_ONCE(*pmdp);
+	if (!pmd_present(pmd))
 		return 0;
 
-	if (pmd_leaf(*pmd)) {
+	if (pmd_leaf(pmd)) {
 #ifdef pmd_page
-		page = pmd_page(*pmd);
-		if (PageHuge(page))
-			return page_size(compound_head(page));
+		if (!pmd_devmap(pmd)) {
+			page = pmd_page(pmd);
+			if (PageHuge(page))
+				return page_size(compound_head(page));
+		}
 #endif
 		return 1ULL << PMD_SHIFT;
 	}
 
-	pte = pte_offset_map(pmd, addr);
-	if (!pte_present(*pte)) {
-		pte_unmap(pte);
+	ptep = pte_offset_map(&pmd, addr);
+	pte = READ_ONCE(*ptep); // gup_get_pte()
+	if (!pte_present(pte)) {
+		pte_unmap(ptep);
 		return 0;
 	}
 
-	page = pte_page(*pte);
-	if (PageHuge(page)) {
-		u64 size = page_size(compound_head(page));
-		pte_unmap(pte);
-		return size;
+	if (!pte_devmap(pte) && !pte_special(pte)) {
+		page = pte_page(pte);
+		if (PageHuge(page)) {
+			u64 size = page_size(compound_head(page));
+			pte_unmap(ptep);
+			return size;
+		}
 	}
 
-	pte_unmap(pte);
+	pte_unmap(ptep);
 	return PAGE_SIZE;
+
+#else /* CONFIG_ARCH_HAS_PTE_SPECIAL */
+
+	return 0;
+#endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
 }
 
 #else